I had to do a lot of Internet searching, but finally came up with some information on the arch I photographed for the post below.
It is called the Solar Arch, and was dedicated in 1985. The artists were William Drexel, who died in 1987, and O. K. Harris. The description, from the Smithsonian Institution Research Information System (SIRIS), is: "An arch with slots cut through the top, so that beams of light illuminate times of the year and hours of the day marked on the sidewalk below the arch."
So my suspicion has been confirmed, and I really must go back to take a better look at this solar calendar.